Why do you need your friend to pin you? BAD IDEA!
If your new to pinz,just get some b12 and practice with that..or google site injections,you'll find its very easy..
Personally having your freind do it isn't the best idea...its best to do it yourself..your "friend" can't feel if he slices next to a nerve,and that injection could be pretty painful for a few days...much better to do it yourself...so you can feel whats going on ,and if you need to pull out and relocate.
That and the fact that will you be able to live with yourself being able to say.."I let my friend stick one in my ass".......I would have to pass on that one...